The engagement control system of the present invention has features that are common to the engagement control system of the automatic transmission shown in U.S. Pat. No. 5,612,874, which is assigned to the assignee of the present invention. The transmission of the '874 patent discloses a planetary transmission having a so-called three-speed Simpson gear set and a simple planetary gear set arranged in series. The two gear sets together establish four forward driving ratios and a single reverse ratio. Pressure-operated clutches and brakes establish each of the forward driving ratios and the reverse ratio as they are applied and released under the control of an electronic processor and hydraulic control valve circuit control system.
A hydrokinetic torque converter is disposed between the crankshaft of an internal combustion engine and torque input elements of the transmission. A neutral state is achieved by disengaging a forward drive clutch, thus interrupting the torque flow path between the planetary gearing and the turbine of the torque converter.
When the output shaft speed is zero and the engine is idling, the forward drive clutch is fully engaged. Thus, the torque converter assumes a stall condition and develops stall torque on the turbine as the impeller of the torque converter continues to be driven by the engine at engine idle speed. This imposes a load on the engine, which effectively results in an undesirable power loss because the engine does not develop useful driving power on the output shaft under such stall torque conditions.
The engagement control system disclosed in the '874 patent uses one variable force solenoid and one on/off solenoid for controlling both forward and reverse engagements. The variable force solenoid communicates with a modulator valve for forward engagements and with a separate modulator valve for reverse engagements. Both of these modulator valves modulate line pressure developed by a main regulator valve in the control valve circuit for producing a clutch pressure that starts from zero pressure to a maximum value, thereby allowing a cushioned engagement of the forward and reverse engagements.
For a forward engagement, the forward clutch of the design of the '874 patent, which establishes a driving connection between the turbine and the input element of the gearing, is controlled. For reverse engagements, a separate high speed ratio clutch is controlled simultaneously with the engagement of a separate reverse brake. Separate engagement control valves are used to distribute either line pressure or clutch pressure to the high clutch or to the forward clutch. This is required in order to satisfy the stall torque capacity that is required during forward and reverse accelerations of the vehicle from a standing start.
The reverse engagement control valve of the design of the '874 patent distributes either reverse line pressure or a reverse modulator pressure to the high clutch during reverse engagement depending upon the output pressure of the on/off solenoid and the variable force solenoid pressure. Thus, there is one pressure threshold for changing between the reverse modulator pressure and the line pressure in reverse drive. Similarly, the forward engagement control valve distributes forward clutch pressure or full line pressure, depending upon the mode of operation of the variable force solenoid for controlling forward clutch pressure. An engagement control valve switches the forward clutch pressure to full line pressure after completion of the forward engagement. This is required since the forward modulator valve must be used, following forward engagement, for controlling upshifts and downshifts between gear ratios. Re-application of forward modulator pressure can be achieved only by moving the manual valve under the control of the operator to a neutral position and then re-engaging the forward clutch.
Furthermore, the system of the '874 patent allows only one distinct pressure threshold for the transition from forward drive modulator pressure to line pressure. Thus, a neutral idle function cannot be achieved since the forward drive clutch cannot be disengaged electronically.
Because only one fixed pressure threshold is available, smooth control of high speed forward and reverse engagement is not possible. Further, since only one variable force solenoid is used to control both the forward and reverse engagements, there is no flexibility on the part of the transmission designer for enhancing static engagement quality. Line pressure amplification and engagement control have different design criteria, but they are achieved with only one variable force solenoid.